Media reports suggest that the chief suspect in the case of American Airlines Flight 587 crash in New York is not the usual one. The cause, combined no doubt with several other factors, may well have been a delinquent we have come to know as a "wake vortex".
A wake vortex is a spinning stream of air, a horizontal whirlpool generated by an aircraft as it moves at high speed through the atmosphere. An aircraft in flight generates at least two of these wake vortices, shed rearwards from the two wing-tips as counter-rotating twins. At high altitude they are relatively harmless; at ground level, however, the twin whirlpools do not spin gently as they do in the free atmosphere, but furiously twist and writhe, interacting with each other and with the ground below.
Sometimes the pair will roll apart, and the process of decay begins. At other times the two combine and reinforce each other, bouncing off the ground, travelling significant distances, and orientating themselves in a variety of unexpected ways. In this mode, they can be surprisingly persistent, their potential destructive power surviving for several minutes after the generating aircraft has innocently proceeded on its way.
In general, the power of a vortex is proportional to the mass of the aircraft that creates it. For this reason, the phenomenon, which was almost unknown before, became a focus of interest in the early 1970s with the advent of wide-bodied aircraft. These could generate vortices with core speeds of several hundred kilometres an hour, accompanied by all the ferocity of a minor hurricane. They could easily flip a light aircraft on its back, and even in the case of larger aircraft, severe wake vortices could cause potentially catastrophic loss of control.
The exact mechanisms which govern the behaviour of wake vortices near the ground are still not fully understood by scientists. The topography of the local terrain is important, but we do not know, for example, how heat radiating from a hot runway affects their development and their persistence, or precisely how they behave when transported laterally by the wind.
The most promising tool for dealing with them is Lidar, by means of which a laser beam, reflected by tiny particles in the atmosphere, provides detailed images of wake vortices as they move across an airfield. In due course, it may be possible to monitor each vortex at a busy airport in this way. For the moment, however, safety can only be ensured by allowing a suitable amount of time to intervene before one aircraft moves directly into the path already taken by another.